By virtue of the tremendous benefits from micro/nanotechnology, implantable and portable devices/systems have become more and more prevalent. However, a major impediment for operating such devices/systems is lack of sustainable and reliable power sources in small scale. Chemical batteries and fuel cells are very bulky and heavy, and can offset the size advantage inherent in micro/nanofabrication technologies. Major disadvantages of using chemical based power sources are the low power density of the fuels as the size of the systems is reduced and the poor performance when they are designed to achieve longer lifetimes. In addition, the requirement of frequent recharging or refueling is an inconvenience and is not favorable for many applications including biomedical implants, space exploration, etc. Instead, solar cells can produce electrical power in a small package without refueling processes and operate in most environments where micro-scale power sources are desired. However, sun-light is always required. Various energy harvesters, which generate electrical power from stray energy like heat and vibration, are simply too weak and provide irregular levels of electric power.
In contrast to the aforementioned power sources, nuclear or radioisotope batteries can provide long lasting power at very high energy density. Radioisotope batteries are devices which convert energy from the decay of radioisotopes into electrical power. These devices come in two varieties: direct conversion and indirect conversion. Direct conversion devices convert the radioactive energy directly to electrical energy via direct-charge generation in dielectrics via ionization or, more commonly, electron-hole pair generation in a semiconductor via excitation or ionization. Indirect conversion devices convert the radioactive energy into an intermediary form of energy, usually photonic or thermal, and convert the intermediary energy form to electricity. Indirect conversion is less efficient but tends to mitigate radiation damage to certain battery components. For any type of radiovoltaic, a radioisotope must be included, as its decay products supply the energy. Thus far, radiovoltaic batteries have been produced by fabricating a device for converting the decay product energy (e.g., semiconductor p-n junction) followed by attaching a radioisotope by hand or electroplating. With these methods, process control is naturally very poor. Radioactive waste is generated as a byproduct of fabrication and human handling of radioactive material is often necessary. Moreover, shortcomings of these cumbersome fabrication processes place strict limitations on the physical dimensions and mass production feasibility of radioisotope batteries. Likewise, the ability to integrate such devices directly into electronic circuitry is limited.
The concept of voltaic cells powered by radioactive decay was first introduced in the late 1950s [1]. A radioisotope (typically a beta-emitter, but sometimes an alpha-emitter) emits radiation that energizes a semiconductor upon impact. When the semiconductor is energized, electron-hole pairs are generated and separated by a built-in electric field due to a p-n or Schottky junction. Betavoltaics and alphavoltaics are similar to solar cells except they use energy from radioactive decay instead of energy from the sun. Because radioactive decay is unaffected by temperature and pressure, a radioisotope micro-power source can operate for extended periods of time and in extreme environments. More importantly, because the energy change in radioactive decay is 104 to 106 times greater than that of a chemical reaction, the energy density (J/kg) of radioactive material is approximately 106 times greater than that of lithium ion batteries [2]. However, it is not easy to increase the capacity due to difficult handling and processing technologies for radioactive materials.
Although nuclear batteries or radioisotope batteries or radiovoltaic batteries or atomic cells have been regarded as promising power sources, their adoption has been extremely limited. Additionally, although the radiovoltaic nuclear battery has been around for about 50 years, the structural design of nuclear batteries has gone relatively unchanged. Instead, nearly all efforts have focused on efficiency and/or lifetime improvement. Researchers have continually attempted to modify the topography of nuclear batteries to improve the directionality of harvesting, thereby increasing efficiency [3], [4]. In addition, radiation-resistant materials such as SiC and Se—S composites have been investigated for their ability to withstand damage to manufacture longer lasting batteries [5-8]. While efficiency continues to improve, the low total power density of nuclear batteries still limits them to niche applications. The current power density of nuclear batteries falls far short of the expectations imposed by the technology's reputation for extreme energy density. Consequently, the applications of nuclear batteries have shifted to almost exclusively micro-scale low consumption devices. In order to transition to widespread use, the issue of power density must be seriously addressed.
While a number of conversion schemes have been developed and introduced thusfar, conventional types of the nuclear battery design have not changed. Specifically, individual cells on thick semiconductor substrates with external radioisotope thick-films are the norm. With this standard fabrication method, battery expansion can only be done laterally or by vertically combining multiple thick substrates. Lateral expansion is undesirable because the lateral dimensions of the device are almost always the largest, leaving little room to expand while still maintaining a small profile. Vertical expansion by combining multiple substrates quickly transitions from micro-scale to macro-scale where the substrates are typically about 300 μm thick. Bonding them together and electrically connecting them also introduces new levels of complexity and difficulty in design and processing. Thus, power density and expansion capabilities are greatly limited.
In addition, nuclear battery fabrication is greatly encumbered by safety hazards and governmental regulations related to human handling of hazardous radioisotope materials. As a result, the fabrication of nuclear batteries remains very complicated, time consuming, and resistant to automation. To avoid hazards presented by vapor-deposition of radioisotopes, nuclear batteries are typically powered by external foils or electroplated thick-films. Introduction of these sources is carried out by hand, and is both hazardous and limiting. Governmental regulations, as well as in-house regulations by health physics committees limit the personnel who can perform these actions and the methods by which they can be done. In addition, attaching external sources to conversion devices is another challenge, and researchers often neglect to address this step [1], [3], and [5]-[8]. This would certainly have to be done by hand, and would be nearly impossible to accomplish on purely micro-scale levels. In addition, post-fabrication attachment of a radioisotope to the device also limits and complicates integration with other technologies.
If nuclear batteries are to be truly integrated with other micro/nano technologies, new methods of fabrication and radioisotope loading must be considered. In view of the foregoing, a need still exists for a method of manufacturing radioisotope batteries that allow for one or more of the following: increased or total automation, decreased battery dimensions, improved precision, improved process control, improved safety, reduced or eliminated production of radioactive water, and improved integration with electronic circuitry.